Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Zilean the Terrorist
Zilean vs Karthus
Grab a dorans and a health pot, feeling mean though? grab boots and some pots.
If you have my runes, you should have more magic penetration than he has magic resistance, just put some bombs on him.
At lvl 1, pick bomb, try to harass, but not to much.
At lvl 2, pick speed, this makes it easier to place your bombs, or get away from him if he traps you.
At lvl 3, pick rewind, try not to use it to much though, you don't want to scare him off and risk your chance of getting a kill.
At lvl 4, don't lvl anything, you want him to think you still have lvl 1 bombs :)
At lvl 5, lvl up ur bomb twice to lvl 3, flash in, ignite(if you have/need) and double bomb him, save speed if you can to get away from his corpse.
You may not have killed him, but he should be wounded and have b'd, allowing you to farm, because you shouldn't be to damaged.
At lvl 6, pick up your ultimate, you should be ahead of Karthus in lvls thanks to your passive, and when he gets his ultimate, try to always have 200 mana so you can use your ultimate in dire situations :)
Double-bombing at early levels takes a lot of mana, if you are intending to harass a lot, pick some mana regen runes or clarity.
For some reason people rarely seem to know what your ultimate does, inform your team if you're solo-queuing. This can also work in your favour since the enemies might try to focus you even with your ultimate on you.